In general, an image captured by an imaging apparatus such as a digital camera or a scanner contains noise. If noise is generated in an object image, quality of the captured image may become degraded.
Conventionally, there has been various noise reduction methods that are mainly based on processing performed using a smoothing filter.
In the object image, intensity of a high-frequency component is generally weaker as compared to that of a low-frequency component, so that noise becomes dominant in a high-frequency region in the image. In such a case, if the smoothing filter which reduces the high-frequency component is applied to the image, the noise can be reduced while maintaining the low to middle-frequency components in the object image.
Further, Japanese Patent Application Laid-Open No. 2006-310999 discusses correcting the high-frequency component in the captured image to become similar to noise, and acquiring an estimate noise image. The noise is then reduced by subtracting the estimate noise image from the captured image. The estimate noise image is acquired by performing correction in which a histogram of pixel values of the high-frequency component in the captured image is caused to become closer to the histogram of the noise in the imaging apparatus.
The above-described noise reduction technique using the smoothing filter reduces the noise and also the high-frequency component in the object image. As a result, a fine texture component of the object image may also become removed.
Further, whether a target pixel is an edge of the object or noise cannot be determined based only on the individual pixel values in the captured image. An edge component of the object image thus becomes included in the estimate noise image according to the technique discussed in Japanese Patent Application Laid-Open No. 2006-310999. As a result, the technique discussed in Japanese Patent Application Laid-Open No. 2006-310999 subtracts the estimate noise image including the edge component from the captured image, so that the edge of the image after performing noise reduction may become blurred.